(1) Field of the Invention
The present invention relates to a hydrolysis stabilizer for ester group-containing resin and a method for hydrolysis stabilization of ester group-containing resin using the hydrolysis stabilizer. More particularly, the present invention relates to a hydrolysis stabilizer for ester group-containing resin, which comprises, as the main component, a carbodiimide having excellent compatibility with ester group-containing resins and thereby showing an excellent anti-hydrolysis effect to ester group-containing resins; as well as to a method for hydrolysis stabilization of ester group-containing resin using the hydrolysis stabilizer.
(2) Description of the Prior Art
Ester group-containing resins (e.g. polyester polyurethane resin) (hereinafter abbreviated to ester type resins, in some cases) are used in various applications for their excellent elasticity and moldability. The polyester polyurethane resins, however, are known to have problems. That is, since they are produced by subjecting a diisocyanate e.g. 4,4'-diphenylmethane diisocyanate (MDI) or tolylene diisocyanate (TDI)! and a bifunctional polyester polyol to urethanization and have a number of ester groups in the molecule, the ester groups are generally hydrolyzed easily by moisture (this causes reduction in molecular weight) and the acidic carboxyl groups generated by the hydrolysis promote further hydrolysis of ester bonds, resulting in strength reduction; owing to this strength reduction, the polyester polyurethane resin becomes incapable of resisting to the strain generated during curing, which invites cracking or further strength reduction.
In order to prevent the properties deterioration of polyester polyurethane, such as mentioned above, attempts have been made to add various additives to a polyester polyurethane resin during its molding to trap the carboxyl groups generated by hydrolysis and prevent the further proceeding of hydrolysis.
As such additives, there are used, for example, oxazoline, epoxy, aromatic polycarbodiimides and monocarbodiimides. Of them, oxazoline and epoxy are not satisfactory because they show a low anti-hydrolysis effect.
The aromatic polycarbodiimides have problems, also. They are a solid and have a high softening point and, in a liquid state when heated, a very high viscosity. Therefore, they have low compatibility with polyester polyurethane resins and cannot be added to the resin during its synthesis; to enable their addition to the polyester polyurethane resin by melt kneading, a very complicated apparatus and a fairly long time are needed and a non-uniform dispersion is formed; as a result, no sufficient anti-hydrolysis effect is obtained.
The monocarbodiimides have problems as well. As the monocarbodiimides, aliphatic or aromatic monocarbodiimides were reported. They have a low melting point and can be added to a polyester polyurethane resin during its synthesis. However, when the polyester polyurethane resin and the monocarbodiimide are dry-blended for their molding or spinning, or when the monocarbodiimide is kneaded into the polyester polyurethane resin from a metering hopper, the monocarbodiimide is separated from the resin owing to the low molecular weight (therefore, relatively high volatility) of the former and, moreover, bleeds out from the resin owing to the low molecular weight; consequently, no intended effect is obtained and the polyester polyurethane resin has a low performance.
In Japanese Patent Application Kokai (Laid-Open) No. 017939/1995 is disclosed a carbodiimide which has a polymerization degree n of 0-10 and isocyanate terminals and which has improved compatibility with polyester polyurethane resins. This carbodiimide, however, has problems when it has remaining isocyanate groups in an amount (hereinafter referred to simply as NCO content, in some cases) of more than 3% by weight (polymerization degree n=10 corresponds to NCO content=about 3.7% by weight, and reduction in polymerization degree n gives an increase in NCO content). That is, in such a carbodiimide, the terminal isocyanate groups give rise to carbon dioxide removal in the presence of moisture and change into urea; when a polyester polyurethane resin containing such a carbodiimide is used as a surface-coating agent or the like, no smooth surface is obtained because of the voids formed by the carbon dioxide removal from the carbodiimide; moreover, since the urea has low compatibility with the polyester polyurethane resin, the polyester polyurethane resin comes to show properties deterioration in elasticity, strength, etc.
Also In the above Japanese Patent Application Kokai (Laid-Open) No. 017939/1995 is disclosed a carbodiimide having terminal isocyanate groups blocked with an alcohol or an amine. This carbodiimide, however, has disadvantages. That is, blocking with an amine produces urea as a product, which is disadvantageous for the reasons mentioned above; use of a blocking agent such as alcohol or amine results in significant increase in carbodiimide equivalents of the carbodiimide, which requires use of carbodiimide in a large amount in order to obtain an anti-hydrolysis effect.